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  A Substitution Mechanism Homework Help - K-12 Grade Level, College Level Chemistry

Introduction to A Substitution Mechanism

A two-step mechanism has been planned for these electrophilic substitution reactions. In the first step, slow or rate-determining, step the electrophile creates a sigma-bond to the benzene ring, producing a positively charged benzenonium intermediate. In the second, fast step, a proton is separated from this intermediate yielding a substituted benzene ring. The following four-part demonstration displays this mechanism for the bromination reaction.

Bromination of Benzene - An Example of Electrophilic Aromatic Substitution

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This technique for electrophilic aromatic substitution should be referred in context with other mechanisms involving carbocation intermediates. These comprise SN1 and E1 reactions of alkyl halides and Brønsted acid addition reactions of alkenes.

To summarize, when carbocation intermediates are created one can suppose them to react further by one or more of the following modes:

1. The cation may bond to a nucleophile to give a addition or substitution product.
2. The cation may transfer a proton to a base, giving a double bond result.
3. The cation may rearrange to a more stable carbocation and then react by mode no.1 or no.2.

SN1 and E1 reactions are respective examples of the first two modes of reaction. The 2nd step of alkene addition reactions proceeds by the first mode, and any of these three reactions may display molecular rearrangement if an initial unstable carbocation is produced. The carbocation intermediate into electrophilic aromatic substitution (the benzenonium ion) is stabilized by charge delocalization (resonance) thus it is not subject to rearrangement. Technically it could react by either mode 1 or 2, but the energetic benefit of reforming an aromatic ring leads to exclusive reaction by mode 2 (that  is proton loss).

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